This invention relates generally to improving fiber quality and process efficiency in thermomechanical and chemi-thermomechanical pulping. More specifically, the invention relates to using specialty chemical compositions including various combinations of a surfactant, a chelant, and other compounds to improve the mechanical properties and brightness of a paper product produced from a pulp material manufactured in such a process. The invention has particular relevance for decreasing freeness and amount of shives, providing energy and chemical savings, and enhancing brightness and mechanical strength of paper products.
Mechanical pulping is a common method to produce inexpensive pulp without a significant loss of mass. Several technologies are currently practiced in mechanical pulping to manufacture products, such as stone ground wood (SGW), pressurized ground wood (PGW), refiner mechanical pulp (RMP), pressurized RMP (PRMP), thermo-RMP (TRMP), thermo-mechanical pulp (TMP), thermo-chemi-mechanical pulp (TCMP), thermo-mechanical-chemi pulp (TMCP), long fiber chemi-mechanical pulp (LFCMP), and chemically treated long fiber (CTLF).
Though purely mechanical pulps have some advantages, such as high opacity, high bulk, and good printing quality, they also have inherent disadvantages, such as low mechanical strength and susceptibility to yellowing. The yellowish color is due partly to formation of chromophoric and leukochromophoric structures in the production process as early as the first refining stage. The light absorption coefficient changes significantly as woodchips are converted into first-stage refined mechanical pulps. The greatest changes occur at wavelengths below 400 nanometers. In the presence of atmospheric oxygen, heat, and/or sunlight structures absorbing light in this region give rise to colored structures. Metal complexes and oxidation reactions may also play a role in creating the increased light absorbency. Avoiding the formation of these structures would result in mechanical pulps with increased brightness and enhanced brightness stability.
Chelants and surfactants (sometimes referred to as surface active agents) have historically had a place in pulp production. Mechanical pulp production is affected by transitional metal ions found in wood, which promote undesirable side reactions including oxidative reactions that cause yellowing. Currently, commodity chelants are used in mechanical pulping processes to immobilize such metal ions. The role of chelants is generally to bind transitional metal cations to prevent their catalytic activity in decomposing bleaching chemicals, such as peroxide, hydrosulfite, and the like. Surfactants have previously been employed in papermaking to accelerate fiber swelling, and to soften and split pulp.
The processes of bleaching and delignification of prepared pulp, but not mechanical pulp manufacturing, have involved combined use of surfactants and conventional chelants. For example, JP 05051889 A2 disclosed use of ethylenediaminetetraacetic acid (“EDTA”) and diethylenetriamine pentaacetic acid (“DTPA”) in oxygen treatment of wood pulp (i.e., delignification). Similar combinations used in ozone bleaching of chemical pulps have also been reported (JP 08188976 A2). Combined use of polymeric chelants and surface-active agents was proposed in JP 07138891 A2 for pulp pretreatment before peroxide bleaching.
Chelant and surfactant combinations have been applied in mechanical pulp production to improve the absorptive capacity of thermomechanical pulp in the course of continuous production from chips (SE 8002027). Pulp brightness, strength, and drainage properties have also been improved by washing woodchips with liquor containing chelants and surfactants between the impregnation and refining stages of the paper production process (See U.S. Pat. No. 5,549,787 and FR 2042117).
Mechanical pulps typically have low strength. Chemical treatment, such as alkalization, is sometimes used to increase strength, at the expense of brightness. There thus exists a need for economical methods of producing mechanical pulp materials having increased mechanical strength and brightness. In particular, it is desirable to develop a cost-efficient mechanical pulp with improved mechanical strength without sulfonation. Preferably, such a development would combine all components in a single composition. Preserving these pulp properties has been difficult without sacrificing printing properties and yield.
Thus there is clear need and utility for system and method for improving fiber quality and process efficiency in thermomechanical and chemi-thermomechanical pulping. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.